Hearing aid technology has progressed rapidly in recent years. Technological advancements in this field continue to improve the reception, wearing-comfort, life-span, and power efficiency of hearing aids. With these continual advances in the performance of ear-worn acoustic devices, ever-increasing demands are placed upon improving the inherent performance of the miniature acoustic transducers that are utilized. There are several different hearing aid styles widely known in the hearing aid industry: Behind-The-Ear (BTE), In-The-Ear or All In-The-Ear (ITE), In-The-Canal (ITC), and Completely-In-The-Canal (CTC).
Generally speaking, a listening device, such as a hearing aid or the like, includes a microphone portion, an amplification portion and a receiver (transducer) portion. The microphone portion picks up vibration energy, i.e., acoustic sound waves in audible frequencies, and creates an electronic signal representative of these sound waves. The amplification portion takes the electronic signal, amplifies the signal and sends the amplified (e.g. processed) signal to the receiver portion. The receiver portion then converts the amplified signal into acoustic energy that is then heard by a user.
Conventionally, the receiver portion utilizes moving parts (e.g., armature, diaphragm, etc) to generate acoustic energy in the ear canal of the individual using the hearing aid or the like. If the receiver portion is in contact with another hearing aid component, the momentum of these moving parts will be transferred from the receiver portion to the component, and from the component back to the microphone portions. This transferred momentum or energy may then cause spurious electrical output from the microphone, i.e., feedback. This mechanism of unwanted feedback limits the amount of amplification that can be applied to the electric signal representing the received sound waves. In many situations, this limitation is detrimental to the performance of the hearing aid. Consequently, it is desirable to reduce vibration and/or magnetic feedback that occurs in the receiver portion of the hearing aid or the like.
U.S. patent application Ser. No. 09/755,664, entitled “Vibration Balanced Receiver,” filed on Jan. 5, 2001, which is a continuation-in-part of U.S. patent application Ser. No. 09/479,134, entitled “Vibration Balanced Receiver,” filed Jan. 7, 2000, now abandoned, the disclosures of which are hereby expressly incorporated hereinby reference in their entirety for all purposes, teaches a vibration balanced receiver assembly designed to establish balanced motion, i.e., equal and opposite momentum of the armature and diaphragm in the assembly and the resulting cancellation of reaction forces inside the receiver portion.
Typically, a receiver assembly comprises an armature that drives reciprocating motion, one or more diaphragms, each of whose reciprocating motion displaces air to produce acoustic output, and one or more linkage assemblies that connect the motion of the armature to the diaphragm or diaphragms. A diaphragm may include a structural element, such as a paddle, that provides the diaphragm with a substantial majority of its mass and rigidity. The paddle is attached to the receiver assembly (aside from its connection to a linkage) by a structure that permits the paddle reciprocating motion to displace air, thereby creating acoustic energy. For example, the paddle may be attached at one of its edges via the structure to some other support member of the receiver. The armature, in contrast, may be attached rigidly to the receiver assembly, so that the motion of the armature involves bending of the armature.
In the case of a vibration balanced receiver, the linkage or linkages connecting the armature and the paddle or paddles may be of a motion-redirection type (such as a linkage, as discussed and described in the afore-mentioned US Patent Applications) so that the velocities of the armature and paddle may be in different directions at their respective points of connection to the linkage. In the context of a motion-redirecting linkage, the method of vibration balancing is to adjust the mass or masses of the paddle or paddles until the total momentum of the diaphragm or diaphragms becomes substantially equal and opposite to that of the armature.
In general, a motion-redirection linkage may either amplify or reduce the magnitude of velocity at its point of attachment to the paddle in comparison to the magnitude of velocity at its point of attachment to the armature. That is, a linkage may constrain the ratio of paddle velocity to armature velocity at a value which is not 1:1, but rather any chosen value within an appreciable range, for example, as high as 10:1 and as low as 1:10. In such cases, since total momentum is the physical quantity to be reduced in the receiver assembly, and since the momentum of a paddle is the product of its mass and velocity, the target value of the mass of a paddle may be different than the mass of the armature. Nonetheless, achievement of a given degree of vibration balancing in a receiver requires that the mass of the paddle must be controlled with precision to a certain value. The masses of diaphragm components other than the paddle or paddles could conceivably also be adjusted, although the characteristics of the other diaphragm components are typically constrained by other acoustic performance requirements. Likewise, the armature mass could conceivably also be adjusted for the purpose of vibration balancing, although once again armature mass is typically not free to be changed in a receiver because that would impact other performance characteristics.
The extent of success of this vibration-balancing method is at least in part reliant on the consistency with which the paddle moves as a hinged rigid body. When a known paddle is used, the vibration-balancing method succeeds only at frequencies below about 3.5 KHz due to insufficient rigidity of the paddle. When the known paddle is driven at higher frequencies, it begins to bend appreciably, especially near 7.5 KHz where the known paddle undergoes a mechanical resonance involving bending of the paddle. This resonant bending changes the proportionality between paddle velocity at the linkage assembly attachment point and the associated diaphragm momentum. The result is an upset of the balance of armature momentum and total diaphragm momentum. The value of paddle resonant frequency (7.5 KHz in the case of the known paddle) is a direct indication of adequacy of paddle rigidity.
The motion-redirection linkage may be realized as a pantograph assembly that utilizes motion of the armature to create motion of the diaphragm that is equal and opposite to that of the armature. The linkage assembly is may be formed from a thin foil because of the low mass, high mechanical flexibility and low mechanical fatigue characteristics that result. The linkage assembly must also satisfy geometric tolerance criteria, both because it must accomplish precise motion-reversal for the purpose of vibration balancing and because it must fit properly between the armature and diaphragm. Early development of the receiver design relied on manually fabrication of the linkage assembly, originally from a photo-patterned foil blank (as shown in FIG. 6A). Through multiple manual folding steps, the diamond leg linkage assembly may be formed (as shown in FIG. 6B). The manual formation of the linkage proved to be unacceptable in terms of throughput and part quality. Due to natural variations inherent to the manual process, unacceptable levels of bending and distortion were present in the majority of the formed piece parts. The manual process throughput was poor due to the high number and complexity of the forming operations required.